1. Field of the Invention
The present invention relates to a method for preparing a latex containing pigments copolymerized with a crystalline polymer, and more particularly, a method for preparing a latex containing pigments copolymerized with a crystalline polymer by which a latex containing pigments copolymerized with a crystalline polymer is prepared in one process without requiring a separate aggregation process, and a low fusing temperature is implemented by substituting a crystalline polymer material having a low melting point of the prepared latex for a mold release agent.
2. Description of the Related Art
A conventional toner used in an electrophotographic imaging apparatus comprises a resin, a pigment, a charge control agent and a mold release agent. Of these components, the resin constitutes about 90% by weight of the total weights of a toner, and plays a role in attaching a toner particle to paper, and the like. A pigment embodies a color, and a charge control agent controls the charges of a toner to assist a toner particle in forming an image efficiently. Finally, a mold release agent enhances the release between a roller and a toner on fusing a toner to prevent an offset and or paper jam.
Such a toner particle may be largely categorized into a pulverized toner prepared by a pulverizing method and a polymerized toner prepared by polymerization. The former is a classical method, and a coloring agent, a charge control agent (CCA) and a mold release agent are melted and dispersed uniformly in a binder resin to obtain a mixture. The mixture is pulverized to a size of a few μm to several tens of μm by a mechanical method, such as an airflow type or a mechanical pulverizer, and the resulting product is classified by size, and then to impart mobility, charge stability, cleanability, and the like, an external additive is added and applied uniformly on a surface.
However, the method of preparing a toner by a pulverizing method has the following disadvantages:                i) the size of the prepared toner particle is difficult to control,        ii) toner particles are very coarse and irregular, and thus, contacting faces between the toner particles becomes larger, thus decreasing mobility, since the shape of the pulverized toner particles is uncontrolled,        iii) the yield is low since size distribution of toner particles is very large, and thus, only toner particles within a specific size range are selected and used,        iv) it is difficult to impart and control electrical properties of the surfaces, due to irregularity of the surfaces,        v) a transferring property and developing property are degraded since the weight distribution of each particle is high, and        vi) a dispersion between a binder resin and an internal additive is difficult.        
Alternatively, the method of preparing a toner by polymerization is a superior method over the pulverizing method. The toners prepared by this method are classified into a suspension-polymerized toner, an emulsion-polymerized toner, and the like. For the emulsion-polymerized toner, which has a particle size of 5 to 15 μm, which is a general size of toner particles, is prepared by preparing an emulsion comprising a resin having a particle size less than 1 μm, a pigment, a mold release agent, and the like, and then adding an aggregating agent thereto to aggregate the emulsion. As described above, a mold release agent is a component of toner particles essential in preventing an offset or paper jam.
Referring to FIG. 1 and FIG. 2, a method of preparing an emulsion-polymerized toner is schematically shown, according to U.S. Pat. Nos. 6,120,967 and 5,863,696.
In FIG. 1, the method includes the operations of preparing an emulsion of mold release agent comprising a polyethylene mold release agent, an anionic dispersing agent, water, and emulsifying the mold release agent using a homogenizer (102); preparing a latex by combining a monomer (styrene plus butyl acrylate plus acrylic acid), an anionic surfactant and a polymerization initiator and polymerizing at 70° C. (104); mixing a pigment and emulsion of the mold release agent and the latex (106); adding an aggregating agent (polyammonium chloride) and aggregating for 30 minutes at 48° C. (108); and melting the aggregated particles by allowing to stand for an hour at 90° C. (110).
In FIG. 2, the method includes the operations of dispersing a pigment by combining the pigment, a dispersing agent (sodium dodecyl sulfate), and distilled water and dispersing for one hour using an ultrasonic homogenizer (202); mixing the pigment dispersion and a monomer by combining the monomer (styrene, butyl acrylate, and methacrylic acid), the pigment dispersion and a dispersing agent (204); polymerizing by agitating for 7 hours at a speed of 500 rpm and 70° C. (206); and filtering and drying (208).
U.S. Pat. No. 6,120,967 recites that an emulsion of a mold release agent is separately prepared, the emulsion is mixed with a latex and a pigment, and an aggregating agent such as a polyammonium chloride is added to a mixture, thus binding together a pigment, emulsion particles of a mold release agent and a latex.
Alternatively, U.S. Pat. No. 5,863,696 describes a process in which a pigment dispersion is first prepared, mixed with a monomer, and then polymerization is performed, thus binding a pigment and a latex without an aggregating agent. However, even in this case, if one is to include a mold release agent, a procedure of aggregating a mold release agent by an aggregating agent by using a separate emulsion of a mold release agent is required.
However, in conventional techniques, when a latex, a pigment and an emulsion of a mold release agent are bound together by use of an aggregating agent, binding even between the same types of particles as well as between different types of particles occurs, and the amounts of a pigment and a wax are difficult to control. Even when a mold release agent is included by using a separate emulsion of a mold release agent, the selection of a dispersing agent that may influence the physical properties of a toner is limited, and preparation of particles less than 1 μm in diameter is difficult.
Furthermore, in conventional techniques, when using a commercial emulsion, the free selection of a mold release agent and an emulsifying agent is limited. Even when using an emulsion of a mold release agent prepared directly, a temperature that is higher than a melting point of a mold release agent and a high speed dispersing apparatus are required, and thus the preparation is difficult, since a conventional mold release agent in a solid state should be dispersed in water in particles less than 1 μm in diameter.